1. Field of the Invention
The present invention relates to a system for the cooling of an anode that can be used to increase the heat dissipation of an X-ray tube operating inside a radiogenic unit without using a heat exchanger comprising a heat-transfer fluid such as air or water placed under forced circulation.
2. Description of the Prior Art
An X-ray tube or radiogenic tube essentially comprises two electrodes, one cathode and one anode contained in a glass tube under vacuum and respectively fixed to the ends of this tube. The cathode is generally constituted by a tungsten filament, housed in a metal part With a shape that is suited for it to play the role of an electronic lens. This metal part shall be called a focusing part. It is designed to send an electron beam focused on the anode. This anode, in the case of rotating anode tubes, is often constituted by a massive disk made of graphite or molybdenum for example, generally covered on one face with a layer of tungsten. Naturally, for special applications, the materials of the anode may be other than those cited here above. The anode may also be constituted by a cylindrical mass made of copper that bears, on its face positioned before the cathode, a plate made of a refractory metal with a high atomic number. It is for the latter type of tube that the present invention is very promising.
When the filament of the cathode is made incandescent and the anode is taken to a positive potential of some volts with respect to the cathode, the electrons emitted by the cathode are accelerated towards the anode by the electrical field created between the two electrodes and bombard a surface of the anode called the focal spot of the X-radiation. This zone of impact of the electrons becomes the main source of X-ray emission throughout the space facing the anode, except at the glancing angles of incidence.
A small proportion of the energy expended to produce the X-ray beam is converted into X-rays while the rest is converted into heat that is stored in the anode. This heat results from the slowing down of the electrons that are emitted by the cathode and strike the anode. The heat is then transferred to the external environment by radiation in the case of a rotating anode tube and by conduction when the anode is fixed.
These X-ray tubes for which the anode is fixed are generally mounted in radiogenic units that furthermore comprise a high-voltage transformer and rectifier elements. The power dissipated by natural convection from the radiogenic unit, which receives all the heat from the anode before dissipating it, cannot permanently exceed 150 to 200 watts. When the anode has to dissipate a higher level of power, of the order of one to four kilowatts, the X-ray tube is placed no longer in the radiogenic unit but in a sheath provided with a water or air cooling system with forced convection, such as a radiator with a ventilator for example.
Such methods of cooling by forced convection have the drawback of requiring an inflow of water from a source, which is difficult when the X-ray tube is mobile around the patient to be radiographed. Or else they have the drawback of requiring the blowing of air in the room in which the radiography apparatus is placed: this room is generally a sterile room in which it is hardly acceptable to stir the air.